#![no_implicit_prelude] cannot coexist with #[rustfmt::skip] and other tool-prelude attributes

[mcy]

#![no_implicit_prelude] disables all preludes, including the tool prelude. However, unlike the std prelude and the extern prelude, there is no way to selectively re-enable it.

This has the concrete result that if I am writing a tool that generates Rust code and want to disable implicit symbols that may happen to clash with my own (hygine, etc), tool-generates symbols, I cannot mark the whole file as #![rustfmt::skip] to disable formatting, since it's machine-generated source code.

See https://godbolt.org/z/bfdKzG47E

[mcy]

There is also the related issue that it is possible to permanently shadow items from the tool prelude: https://godbolt.org/z/P5eYn69hz

Amusingly, the expected fixes do not work, because, again, these names are not real:

#[rustfmt::skip]
fn works() {}

#[::rustfmt::skip]
fn doesnt() {}

use rustfmt as x;
#[x::skip]
fn also_doesnt() {}

[PatchMixolydic]

This even affects tools that are explicitly declared with #![register_tool] (playground):

#![feature(register_tool)]
#![no_implicit_prelude]
#![register_tool(a)]

#[a::skip]
fn main() {}

error[E0433]: failed to resolve: use of undeclared crate or module `a`
 --> src/main.rs:5:3
  |
5 | #[a::skip]
  |   ^ use of undeclared crate or module `a`

[jyn514]

cc #66079